I have outside the regular (i.e. non-GFCI) power outlet that you see on the left.
It has a nice double-cover that will keep water out.
I am about to replace it with a GFCI outlet (right of the picture). The GFCI outlet will not accept the receptacle you see on the left, because there is no room for the screw in the center. That space is occupied by the test/reset buttons.
Is there any disadvantage in installing the ordinary plastic cover (bottom right of the picture) to cover a GFCI outlet outdoors? It is vertical and is under a 3ft shed, so it will somewhat be protected from rain.
Best Answer
An unmentioned part of the question is the fact that the OP is trying to combine components of two incompatible electrical systems: the newer Decora system, and the older system it replaces (I don't know the name...)
The Decors outlet in question does not lack a central screw hole because of the GFC buttons. Such a central hole is not found on any Decors outlet. It is simply not part of the system design. The screws holding a Decors cover plate engage in the central holes in the tabs that connect the component to the electrical box.
There are Decora style weatherproof cover plates; here is one example:
Use one of these with your Decors style outlet
Of course, the concerns expressed by others about the effectiveness of this solution in resisting moisture still apply...